This invention relates to glazing systems for providing a heat gain in an enclosed space through the greenhouse effect. In such an enclosure, the glazing system permits a substantial part of the spectrum of sunlight to enter the enclosure, where it is partially converted to infrared radiation. The glazing system is relatively opaque to infrared, and therefore traps heat energy within the enclosure. The enclosure also reduces heat loss by preventing convection of air through the glazing. The glazing system of the present invention is particularly useful in novel greenhouse structures, but it may be used in numerous other structures such as swimming pool enclosures, sun rooms, cold frames, and the like. The system may be used on numerous structures having frames of wood, steel, aluminum, fiberglass, metals, or other materials, or on structures having novel frames.
Present greenhouses are glazed with glass sheet or plastic film or sheet.
Glass-glazed greenhouses were used universally until about forty years ago. They are expensive to build and operate, are subject to breakage in hail storms, and require heavy permanent footings. Although the glass glazing transmits photosynthetically active radiation (PAR) well, the framing members required to support the heavy glass reduce the overall light reaching the plants in the greenhouse. For these reasons, they are increasingly being replaced by greenhouses having a plastic film glazing.
The standard plastic being used today for greenhouse glazing is polyethylene film. The film is stapled, nailed, taped, tied, and attached by other locking systems to frames ranging from wood to steel and aluminum. Because polyethylene film is relatively inexpensive, its use has become widespread to the point of overwhelming dominance, particularly in commercial greenhouses where appearance is not a major concern.
Despite its popularity, the use of polyethylene as a glazing for greenhouses has severe disadvantages. The average life span is approximately two years. It is an inherent characteristic of the polyethylene film to begin degrading as soon as it is manufactured. Solar ultraviolet radiation causes polyethylene to discolor and to become brittle. One of the largest concerns to customers of these systems is the fear of losing one's crops. This concern is heightened by the polyethylene film's becoming less flexible and more brittle in cold weather, so that it is most likely to fail during cold, windy weather, when its failure will cause the greatest damage to the crops in it. Because of embrittlement, the film becomes very difficult or impossible to repair when damaged. The older the film, the more brittle and the more difficult to repair. Because of the discoloration due to ultraviolet rays, light transmission, and in particular PAR transmission, also diminishes and worsens with exposure and age. This has significant effects on plant growth. As the film becomes more brittle, it also loses abrasion resistance characteristics and general strength characteristics.
Because of the short life of polyethylene films, greenhouse design is now largely dictated by the necessity of changing the film glazing easily. Therefore, many features which would be desirable in a greenhouse are omitted or degraded.
The polyethylene film has a relatively high tensile modulus, but once stretched it is permanently deformed. Therefore, with time the film loses tautness and tends to flap.
Because greenhouses tend to have high internal humidity, condensation on the film tends to drip; this is detrimental to many plants and annoying for workers. Condensate droplets also may interfere seriously with the optical functioning of the greenhouse, by blocking PAR and transmitting IR. Special coatings are sometimes provided to increase sheeting of condensate and decrease dripping, but these add cost and may interfere with the optical characteristics of the film.
Polyethylene films also tend to be much more transmissive of IR than is glass. Therefore, they are not as efficient in terms of the greenhouse effect in converting sunlight to heat and trapping the heat energy.
The well-known drawbacks of polyethylene film glazing have led to intense efforts to find better glazing materials, but no commercially acceptable alternatives have been found.
Other plastic films and panels have been tried, but have not met all of the criteria of a successful greenhouse glazing such as strength, resistance to sunlight and weathering, lack of cold weather embrittlement, abrasion resistance, resistance to condensate dripping, ease of fabrication, and proper optical characteristics, such as transmission of light in the region known as photosynthetically active radiation (PAR), optical clarity in the visible region, and opacity in the infra-red (IR) region to trap heat energy. The better materials have also been very expensive. For example, among the sheet materials, acrylic is brittle and must be used in sheets having a thickness much greater than a polyethylene film. It is consequently expensive and is limited in design applications. Polystyrene degrades quickly in sunlight. Polycarbonate is even more expensive and unwieldy than acrylic. Fiberglass (glass fiber reinforced polyester or polyester/acrylic blend) discolors and has markedly inferior optical characteristics. Among the films, polyvinylchloride (PVC) must be heavily plasticized for flexibility, and the plasticizer migrates to the surface with heat and aging, where it collects dirt and diminishes light (PAR) transmission.
Although the performance of a glazing material can be estimated by measurement of its physical and optical characteristics, so many variables are involved in a real greenhouse that performance can be truly determined only by growing plants in a greenhouse using the glazing material.